RESUMO
Magnetically induced catalysis using magnetic nanoparticles (MagNPs) as heating agents is a new efficient method to perform reactions at high temperatures. However, the main limitation is the lack of stability of the catalysts operating in such harsh conditions. Normally, above 500 °C, significant sintering of MagNPs takes place. Here we present encapsulated magnetic FeCo and Co NPs in carbon (Co@C and FeCo@C) as an ultrastable heating material suitable for high-temperature magnetic catalysis. Indeed, FeCo@C or a mixture of FeCo@C:Co@C (2:1) decorated with Ni or Pt-Sn showed good stability in terms of temperature and catalytic performances. In addition, consistent conversions and selectivities regarding conventional heating were observed for CO2 methanation (Sabatier reaction), propane dehydrogenation (PDH), and propane dry reforming (PDR). Thus, the encapsulation of MagNPs in carbon constitutes a major advance in the development of stable catalysts for high-temperature magnetically induced catalysis.
RESUMO
Heating magnetic nanoparticles with high frequency magnetic fields is a topic of interest for biological applications (magnetic hyperthermia) as well as for heterogeneous catalysis. This study shows why FeC NPs of similar structures and static magnetic properties display radically different heating power (SAR from 0 to 2 kW g-1). By combining results from Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS) and static and time-dependent high-frequency magnetic measurements, we propose a model describing the heating mechanism in FeC nanoparticles. Using, for the first time, time-dependent high-frequency hysteresis loop measurements, it is shown that in the samples displaying the larger heating powers, the hysteresis is strongly time dependent. More precisely, the hysteresis area increases by a factor 10 on a timescale of a few tens of seconds. This effect is directly related to the ability of the nanoparticles to form chains under magnetic excitation, which depends on the presence or not of strong dipolar couplings. These differences are due to different ligand concentrations on the surface of the particles. As a result, this study allows the design of a scalable synthesis of nanomaterials displaying a controllable and reproducible SAR.
